This application claims priority under 35 U.S.C. xc2xa7119 and/or 365 to Appln. No. 199 52 885.3 filed in Germany on Nov. 3 1999; the entire content of which is hereby incorporated by reference.
The invention relates to a method for operating a power plant, which contains a gas turbine set.
It is known to utilize the high exhaust-gas temperatures of gas turbine sets for the generation of synthesis gas. In this context, water is evaporated in a waste-heat boiler by means of the exhaust-gas heat of the gas turbine set. The steam thus obtained is intermixed, in a reactor likewise heated by the exhaust gas of the gas turbine, with a hydrocarbon-containing crude fuel, for example natural gas. When the temperature is sufficiently high, a low-calorie synthesis gas is obtained in the reactor from the crude fuel and the steam and contains as essential constituents the components comprising steam, hydrogen, unconverted crude fuel, carbon dioxide and carbon monoxide. A reactor gas emerges from the reactor, which reactor gas consists, in a varying composition, of synthesis gas, crude fuel and unconsumed steam and which can be burnt in a combustion chamber of the gas turbine set. The combustion of such low-calorie gases affords advantages in terms of pollutant emissions, in particular of nitrogen oxides, since the flame temperatures are lower. The high hydrogen fraction at the same time ensures stable combustion. The large fraction of ballast materials in the reactor gas results in high fuel mass flows and therefore high specific power outputs of the gas turbine set.
Power plants of this type with chemical recuperation of the exhaust-gas heat offer high efficiencies. However, a thorough conversion of crude fuel and steam into synthesis gas requires a comparatively high temperature level. This is afforded, at least approximately, precisely in the exhaust gas of modern gas turbine sets with sequential combustion which have a plurality of combustion chambers at different pressure stages. Additional firing upstream of the waste-heat boiler which may be necessary in some cases in order to achieve the necessary reactor temperature can have small dimensions. This makes the practical use of chemical recuperation of the exhaust-gas heat in gas turbine sets with sequential combustion attractive.
The object on which the present invention is based, in a power plant in which the exhaust-gas heat of a gas turbine set is utilized for the generation of synthesis gas, is to utilize in the best possible way the potentials of such an operation method for efficiency and power output.
It is therefore proposed, according to the invention, that a method for operating a power plant, which power plant contains at least one gas turbine set with at least one combustion chamber of a highest pressure stage and with at least one combustion chamber of at least one lower pressure stage, and in which method a hot gas is first expanded from a combustion chamber of a high pressure stage in a turbine, at the same time delivering mechanical power, and is introduced into a combustion chamber of a lower pressure stage, an expanded hot gas of the gas turbine set flowing through a waste-heat boiler, in which waste-heat boiler a steam quantity is generated, this steam quantity being introduced into a reactor installed in the waste-heat boiler, into which reactor a quantity of a hydrocarbon-containing crude fuel continues to be introduced, and a reactor gas consisting at least partially of synthesis gas being generated in the reactor from the steam quantity and from the crude fuel, which reactor gas is burnt at least partially in at least one combustion chamber of the gas turbine set, be designed in such a way that the synthesis gas is introduced, at least for a predominant part, into a combustion chamber of as high a pressure stage as possible and is burnt there. Combustion chambers of lower pressure stages are fired preferably directly by means of the crude fuel.
The essence of the invention is, therefore, in a power plant with a waste-heat boiler, to design the waste-heat boiler initially in a way known per se as a reactor for the generation of synthesis gas. Modern gas turbine sets with sequential combustion, which have at least two combustion chambers operating at different pressures, are particularly suitable for the generation of synthesis gas in a waste-heat boiler because of the high exhaust-gas temperatures which can be achieved. Steam is generated in the waste-heat boiler and is combined with a hydrocarbon-containing crude fuel in a reactor heated by the exhaust gas. A reactor gas is obtained there, which, when the reactor temperature is sufficiently high, consists essentially of hydrogen, carbon dioxide, carbon monoxide and steam not consumed for the generation of synthesis gas and, when the reactor temperature is comparatively low, of unconverted crude fuel, the inert steam constituting a ballast component of the reactor gas. According to the invention, in a gas turbine set with sequential combustion, the reactor gas is introduced, at least for a large part, into the combustion chamber of the highest pressure stage, insofar as the gas quantity is capable of being converted there. Combustion chambers of low pressure stages are operated preferably directly by means of a crude fuel which does not necessary have to be identical to the crude fuel supplied to the reactor for the generation of synthesis gas, but in the majority of cases will be because of practical considerations. The entire generated reactor gas quantity is introduced preferably into the combustion chamber of highest pressure and is burnt there, as long as the production rate of the reactor gas does not exceed the gas quantity capable of being utilized in the combustion chamber of highest pressure. The reactor gas is thus introduced at the highest possible pressure into the working process of the gas turbine set. The efficiency of a power plant with chemical recuperation of the exhaust-gas heat which is operated according to the invention far exceeds that of a power plant with steam injection and comes very close to that of a combined cycle plant in which the apparatus is appreciably more complicated.
Optimization of the efficiency of a chemically recuperated gas turbine set with sequential combustion is therefore achieved, with the exhaust-gas heat utilized as fully as possible, the generated reactor gas is burned as far as possible in the combustion chamber of the highest pressure stage. When, for example, methane or natural gas with a high methane content is used as crude fuel, it proves beneficial, in this respect, if possible to set the variable mass ratio of steam and crude fuel at a value of 5 parts of steam to one part of methane. When the hot-gas temperature in the combustion chamber of the highest pressure stage reaches a specific limit value, which is determinedxe2x80x2, for example, by the permissible material temperatures of the components in the hot-gas path or, by the operating concept of the gas turbine plant, no further fuel can be utilized in the combustion chamber of the highest pressure stage. Reactor gas generated beyond the quantity capable of being utilized in the combustion chamber of the highest pressure stage is introduced into a combustion chamber of a lower pressure stage, preferably into a combustion chamber of the next lower pressure stage, and is burned there. Thus, in a further operating state, the combustion chambers of the two highest pressure stages are operated with reactor gas, specifically the combustion chamber of the highest pressure stage exclusively or for a predominant part and the combustion chamber of the second highest pressure stage for the fraction to be covered by the production of reactor gas and for the rest with crude fuel, while, where appropriate, further combustion chambers of lower pressure stages are operated at least predominantly with the crude fuel so as to operate with optimum efficiency. Reactor gas which can no longer be utilized even in the combustion chamber of the second highest pressure stage may, if present, be utilized in a combustion chamber of a third highest pressure stage. In this way, an increasing production of reactor gas is supplied in stages, at an ever lower pressure, to the working process of the gas turbine plant, and a maximum fraction of the generated reactor gas is always used at the highest pressure stages. There are, of course, limits regarding the capability of using the reactor gas for operating the gas turbines set. When more reactor gas is generated overall than can be burned in the combustion chambers of the gas turbine set at a given power output of the power plant, the excess reactor gas can also be utilized for operating other firing plants. However, along the lines of the actual idea of the invention, this option should be utilized,, in fact, only when a high reactor gas production made possible by the exhaust-gas heat can, in fact, no longer be utilized within the actual working process of the gas turbine set. The highest efficiency of the power plant is achieved, in fact, when the reactor gas produced with the exhaust-gas heat being fully utilized can be burned completely in the combustion chamber of the highest pressure stage, and the teaching of the present invention is also to be seen primarily in this. All the above, described diversions of reactor gas into lower pressure stages are to be considered as secondary, and, with a view to efficiency optimization, the aim must always be for the generated reactor gas to be converted as completely as possible in the combustion chamber of the highest pressure stage.
This process management is also to be preferred for reasons of reaction kinetics. In a gas turbine set with sequential combustion, the greatest temperature increase usually occurs in the first combustion chamber, that is to say the combustion chamber of highest pressure. The high heat release rate results in a high potential for the formation of nitrogen oxides. In the combustion of the low-calorie reactor gas with high ballast fractions, local temperature peaks are effectively prevented, thus leading to combustion low in nitrogen oxides. At the same time, a high hydrogen fraction in the synthesis gas fraction of the reactor gas contributes very decisively to stabilizing the comparatively cool flame. By contrast, in the combustion chambers of lower pressure stages, the working medium is simply reheated. Due to the comparatively low temperature increase, the formation of nitrogen oxides in these combustion chambers is in any case only slight. Moreover, such combustion chambers are often designed as self-ignition combustion chambers. That is to say, the ignitability of the fuel used must be sufficiently high to ensure that spontaneous self ignition of the fuel takes place in the hot gas. Low-calorie reactor gas with high fractions of carbon monoxide and of inert gases does not readily ensure this; in this case a pilot flame may have to be generated by means of a highly ignitable fuel, such as diesel oil, in order to ensure reliable self ignition of the reactor gas.
The operating method according to the invention also offers advantages precisely when the power plant is to be operated with liquid crude fuels for reasons of availability. It is known that, without appropriate measures, the production of nitrogen oxides is substantially greater in operations with liquid fuels than in operations with gaseous fuels. It proves advantageous, in this case, to process part of the crude fuel into synthesis gas which is burnt as the sole fuel in the combustion chamber of the highest pressure stage. In this way, as described above, in a combustion chamber of the highest pressure stage, which, in the case of the gas turbine sets with sequential combustion actually used nowadays, has the highest thermal power output at temperature increases beyond 700xc2x0 C., a low-calorie gaseous fuel is burnt, this having the positive effect described on the formation of nitrogen oxides. A combustion chamber of a lower pressure stage is fired at a comparatively appreciably lower level; the temperature increases are restricted, for example, to 200xc2x0 C. On account of the markedly low thermal power output, the tendency of the atmospheric nitrogen to oxidate is only slight, and because of this, even in the case of firing with liquid fuel, such high production rates of atmospheric pollutants do not occur. Furthermore, the use of highly ignitable fuels, such as diesel oil, is advantageous precisely in a self-ignition sequential combustion chamber of the well-known type. In an operating method according to the invention, therefore, a power plant can be operated with a single crude fuel and fuels of the kind suitable in each case are nevertheless employed at different points. Precisely for the operating mode described above, it is highly beneficial if, as described below, additional firing is operated in the waste-heat boiler, in order to ensure a sufficient synthesis of reactor gas from the liquid fuel in the entire operating range. To be precise, in this case, the crude fuel can be converted to 100% into synthesis gas under all circumstances.
It must be stated, in summary, that as full a utilization as possible of the reactor gas in the combustion chamber of the highest pressure stage, normally the first combustion chamber, of a gas turbine set with sequential combustion is to be preferred on account of thermodynamic considerations and questions of reaction kinetics.
In part-load operation and, in particular, during the startup of a chemically recuperated gas turbine set, operating states will arise in which the available exhaust-gas heat is not sufficient to generate a reactor gas quantity sufficient for operating the combustion chamber of the highest pressure and having a desired synthesis gas content. In this case, the combustion chamber of the highest pressure must be fired directly, at least partially, with the crude fuel. With an increasing production of synthesis gas, the crude fuel is replaced successively by synthesis gas. It should also be stated that not only the overall availability of heat quantity from the exhaust-gas heat is critical for the production of reactor gas, but a minimum temperature is also necessary in order to generate synthesis gas. Consequently, for carrying out the method according to the invention, a gas turbine set is advantageously used which has at least one adjustable row of compressor blades, in particular an adjustable preguide row. By virtue of the adjustment of the compressor blading, the mass flow of the working medium of the gas turbine set can be varied within a particular range, and therefore also the exhaust-gas temperature upon exit from the gas turbine and upon entry into the waste-heat boiler. When the method according to the invention is carried out, adjustable compressor blades can be used in such a way as to regulate the exhaust-gas temperature in part-load operation to a target value such that, even when the load of the power plant is low, a specific exhaust-gas temperature conducive to the production of synthesis gas is reached.
The desired reactor temperature and therefore the desired temperature of the exhaust gas upon entry into the waste-heat boiler are determined by the desired degree of conversion of the crude fuel into synthesis gas. In synthesis gas reactors which are conventional according to the prior art, a substantial conversion of, for example, methane is achieved only at temperatures beyond approximately 850xc2x0 C. However, such high temperatures in the exhaust gas of a gas turbine set would lead to considerable strength problems in the region of the last rows of turbine moving blades. If, therefore, it is desirable to have as complete a conversion as possible of the crude fuel into synthesis gas, for example, in the above-described operating method with diesel oil as crude fuel, it is necessary to use additional firing in the exhaust-gas path upstream of the waste-heat boiler in order to raise the reactor temperature. Even if additional firing may lead to slight losses in the efficiency of the plant as a whole, this can be more than compensated for by operational benefits. As described above, additional firing may afford considerable advantages precisely during the startup and in the lower part-load range of the power plant, but, irrespective of the exhaust-gas temperature of the gas turbine, a specific reactor temperature can be ensured. Furthermore, additional firing may be utilized in order to carry out a quick-reacting regulation of the reactor temperature, in particular in transient operating states with fluctuations of the exhaust-gas temperature.
When additional firing is employed during the startup of the power plant, that is to say the gas turbine set is still at a standstill or at a very low rotational speed, it is necessary to introduce air into the exhaust-gas tract of the gas turbine upstream of the additional firing. For this purpose, when the power plant is being started up, an additional-air flap is opened, and an additional-air blower conveys ambient air to the additional firing and from there into the waste-heat boiler.
The optimization of the efficiency of the overall plant necessitates, of course, as full a utilization of the exhaust-gas heat as possible. The exhaust gas is therefore to be cooled, upon exit from the waste-heat boiler, to a lower limit temperature which is determined, for example, by the dew point of exhaust-gas components. In order to set the exhaust-gas final temperature at this target value, it proves expedient, in particular, to vary the steam quantity degenerated in the waste-heat boiler. In this case, it is particularly advantageous that the ballast content, that is to say, in particular, the steam content of the generated reactor gas can be varied within wide ranges and may reach comparatively high values. It is therefore perfectly possible, for example when methane is used as crude fuel, to supply four or even five times the quantity of steam to the reactor when this is expedient for utilizing the exhaust-gas heat as effectively as possible. In other words, the utilization of the exhaust-gas heat can be influenced, on the one hand, by a control of the quantity of the generated reactor gas and, on the other hand, by a control of the reactor gas quality. Furthermore, it is, of course, also possible not to utilize part of the steam generated in the waste-heat boiler for the generation of reactor gas, but instead to supply this steam to other consumers. Thus, excess steam can be introduced into the gas turbine process at a suitable point and be used for cooling components of the gas turbine set which are subjected to high thermal load, or it could even be utilized in a steam turbine.
The above-described variation of the quantity supplied to the reactor in relation to the quantity of the crude fuel and the possibility of influencing the rate of conversion of the crude fuel into synthesis gas via the reactor temperature afford wide-ranging possibilities of setting in a controlled manner fuel parameters, such as the calorific value or the flame front velocity of the reactor gas, as required.
The explanations show that the method according to the invention makes it possible to have an entire series of embodiments. The selection of a specific variant will depend, in practice, to a great extent on the desired operating mode and the available crude fuel and also on thermodynamic boundary conditions predetermined by the gas turbine set to be used. The fundamental inventive idea common to all the variants is to fire the reactor gas as completely as possible in a combustion chamber of a highest pressure stage for the purpose of maximum exergy utilization.